THE HYDRATES OF DOUBLE SELENATES

This study deals with the hydrates of double selenates of the type M2( I)M(II)(SeO4)2 . XH2O (x = 2 or 6) from the point of view of the effec t of the M(I)(M(I)= K, Rb, Cs, Tl, NH4) and M(II) (M(II) = Mg, Zn, Cd, Mn, Co, Ni, Cu) cations on the water molecules in-the coordination sp here of the bivalent metal. The bonds of the water molecules were stud ied on the basis of the bands of the ''wagging'' libration modes of th e water molecules, and the bands of the stretching vibrations in the i nfrared molecular spectrum, and using thermal analysis. According to t he study of ''wagging'' libration modes the deformation of the coordin ation sphere of the hexaaqua cations [M(II)(H2O)6]2+ increases with th e decreasing radius of cation M+ and with the increasing radius of cat ion M2+, and thus the tendency to form dihydrates of the studied doubl e selenates also increases. This dependence corresponds with the therm al decomposition of the hexahydrates to dihydrates. Thermal decomposit ion of dihydrates to the anhydrous salts is determined primarily by th e strength of the hydrogen bonds between the test anions and the water molecules, once again affected by the two types of cations. The NH4+, Tl+, Cu2+, and Ni2+ cations specifically affect the thermal decomposi tion of the hydrates of the double selenates.